This invention relates to tape joint compounds, spachtling (or spackling) compounds and similar wall repair compositions and is especially concerned with inexpensive gel-resistant glass bubble-filled compositions of this type.
During installation, plastered surfaces frequently develop cracks, pits, etc. When plaster wallboard is mounted, there are inevitably nail dimples at the location where the individual sheets are nailed to studs or ceiling joists. Perforated tape is commonly applied over the joint between two adjoining gypsum bond panels, joint repair compositions being applied both under and over the tape. Even if the original installation is substantially free of defects, shrinkage or uneven settling is likely to cause cracks at a later time. If pictures are moved from one location to another, nail holes remain at the first location. Flaws of all these types are typically repaired with a joint-filling or spachtling compound, which is a spreadable composition that is applied and allowed to dry or cure. Such compounds typically include polymeric binder, filler, water, water-retention agents, etc.
In recent years, wall repair compositions of the type just discussed have also included a substantial volume of glass microbubbles, reducing the weight of the composition, making it easier to spread smoothly, and providing a surface that is more easily sanded smooth after the composition dries; see, e.g., U.S. Pat. Nos. 3,386,223, 4,086,098, 3,183,107, and 4,391,647.
One method of preparing glass microbubbles is taught in U.S. Pat. No. 3,030,215, which describes the inclusion of a blowing agent in an unfused raw batch of glass-forming oxides. Subsequent heating of the mixture simultaneously fuses the oxides to form glass and triggers the blowing agent to cause expansion. U.S. Pat. No. 3,365,315 describes an improved method of forming glass bubbles in which pre-formed amorphous glass particles are subsequently reheated and converted into glass bubbles. U.S. Pat. No. 4,391,646 discloses that incorporating 1-30 weight percent of B.sub.2 O.sub.3, or boron trioxide, in glasses used to form microbubbles, as in U.S. Pat. No. 3,365,315, improves strength, fluid properties, and moisture stability. A small amount of sodium borate remains on the surface of these bubbles, causing no problem in most applications. Removal of the sodium borate by washing is possible, but at a significant added expense; even where washing is carried out, however, additional sodium borate leaches out over a period of time.
Because of its ready availability and low cost, polyvinyl alcohol (PVA) is a desirable binder for use in spachtling compounds or other wall repair compositions. Unfortunately, however, when a wall repair composition contains polyvinyl alcohol and borate-containing glass bubbles, the borate reacts with the hydroxyl groups on the polyvinyl alcohol, promptly gelling the composition and rendering it unusable. To a lesser extent, the same problem arises when polyvinyl acetate emulsion, which contains a certain amount of polyvinyl alcohol, is employed as the binder. Guar gum, a naturally occurring high molecular weight polyhydroxy compound, similarly gels in the presence of borate. These and other high molecular weight polyhydroxy compounds may also be used for water retention viscosity control, latex stabilization, etc. Gelation may result in all such cases. Although gelation may be counteracted by acidifying the system, other problems typically result; for example, such common fillers as calcium carbonate dissolve in acidic systems, releasing carbon dioxide and thus causing intolerable bubbling. Prior to the present invention, then, no way of coping with the gelation problem has been recognized.